Liquid catalysts offer many advantages over conventional solid-supported catalysts. Unsupported liquid catalysts require less equipment and raw materials to make them and impart fewer impurities to the final polymer product. The activity of a liquid catalyst is not adversely influenced by the surface area of a support material. Additional advantages are encountered when a high-activity unsupported metallocene catalyst is used for polymerizations in a fluidized bed reactor.
The use of liquid or unsupported catalysts are disclosed in U.S. Pat. No. 5,317,036. This patent is directed to the use of unsupported, soluble olefin polymerization catalysts, particularly metallocenes in liquid form, in gas phase reactions. These catalysts have a droplet size in the range of about 1 to about 1,000 microns.
However, when a liquid catalyst is employed in gas phase polymerization, several phenomena can occur. First, the soluble or liquid catalyst tends to deposit on the resin or polymer forming the fluidized bed which in turn leads to accelerated polymerization on the surface of the particles of the bed. As the coated resin particles increase in size, they are exposed to a higher fraction of catalyst solution or spray because of their increased cross-sectional dimensions. If too much catalyst is deposited on the polymer particles, they can grow so large that they cannot be fluidized thereby causing the reactor to be shut down.
Second, using liquid catalyst under conditions of high catalyst activity, e.g., a liquid metallocene catalyst, the initial polymerization rate is often so high that the newly formed polymer or resin particles can soften or melt, adhering to larger particles in the fluidized bed. This needs to be avoided or minimized to avert reactor shutdown.
On the other hand, if the polymer particles size is too small, entrainment can occur resulting in fouling of the recycle line, compressor, and cooler and increased static electricity leading to sheeting can result.
Accordingly, there is a need to control growth in polymer particle size when using a liquid catalyst in a gas phase fluidized polymerization process.